A new technique for acquiring channel plans in wavelength-division multiplexing (WDM) systems is disclosed. This technique allocates channels from available channel slots by minimizing four-wave mixing (FWM) crosstalk and taking into account fiber characteristics. It can provide channel plans for WDM systems in single-mode fiber with a non-uniform dispersion profile along the transmission path.
With the wide deployment of dense wavelength-division multiplexing (DWDM) systems, intensity-dependent nonlinear effects in low dispersion optical fiber have become a significant issue, in which four-wave mixing (FWM) is of particular concern due to the large number of channel counts and narrow channel spacing in the DWDM systems. To reduce the FWM effect, channel plans with unequal channel spacing have been used. These channel plans can be classified into two categories: the zero-FWM channel plans; and the non-zero-FWM channel plans. The zero-FWM plans do not have any FWM products falling onto signal channels and thus significantly reduce FWM effects. However, to achieve this, substantial usable bandwidth is sacrificed and extremely low bandwidth efficiency results. Moreover, if the given channel slots are fixed to ITU grids, the number of channels in a zero-FWM channel plan is quite limited.
The non-zero-FWM plans allow some FWM products to fall onto the signal channels, but with maximum level of FWM crosstalk is less than the required limit. These channel plans obviously have clear advantage over the zero-FWM plans in realizing that it is not necessary to achieve zero FWM in an actual WDM systems as long as the FWM crosstalk level does not limit the system performance. The bandwidth efficiency is dramatically increased in non-zero FWM channel plans. However, due to the large number of channels and the complex relation between channels and FWM products, obtaining an optimum unequally spaced non-zero-FWM channel plan is not an easy task. If the fiber characteristics are considered in the procedure, the problem becomes even more complex.
A periodic allocation method has been proposed to obtain the non-zero-FWM channel plans. In this method, the entire available optical bandwidth (usually limited by the optical amplifiers) is divided into multiple sub-bands. In each sub-band, a zero-FWM channel plan is obtained. A guard band is allocated between adjacent sub-bands to reduce FWM produced by channels in different sub-bands. The FWM crosstalk is significantly reduced in channel plans obtained using this method compared to the equally spaced channels, because no FWM crosstalk is produced inside each sub-band and also a large separation exists between channels in different sub-bands. However, this method is far from optimum since it does not consider fiber dispersion at different channel wavelengths. In addition, the periodic allocation method is counter-intuitive since a larger dispersion region should have more channels and a zero-dispersion region should have less channels.
Recently, another method has been proposed to obtain unequally spaced channel plans with non-zero FWM. This method minimizes the FWM products that fall onto signal channels under constraint of a given bandwidth expansion ratio. A sequence is produced which represents the channel spacing between channels in the desired channel plans. The optimization is performed by manipulating this sequence to reduce the number of FWM products. The advantage of this method is that the resulting channel plan satisfies the required bandwidth expansion factor. However, a drawback associated with this method is that this expansion factor usually is quite large, (1+N/2) for an N-channel plan. Also, the resulting channel plan may not satisfy the required FWM limit. Moreover, this algorithm does not consider the fiber characteristics, i.e., fiber dispersion. Minimizing FWM products does not necessarily minimize the FWM crosstalk since the FWM efficiency strongly depends on the fiber dispersion and channel spacing.
An exhaustive computer search may be another choice for selecting a channel plan. However, due to the large number of channels in DWDM systems, the possible combinations of the channels in forming a channel plan is so large that it is prohibitive to use this approach. For example, if a 40-channel plan is selected from a 96-channel WDM system, the number of possible choices is 1.3xc3x971027. Clearly, it is impossible to try every combination to find the optimum channel plan.
Upgrading an existing WDM system presents additional challenges for channel allocation. With the dramatic increase in demand for transmission capacity from, for example, internet applications, the channel counts of WDM systems have correspondingly increased. Transmission systems have thus evolved from normal WDM systems to dense WDM systems, and to current ultra-dense WDM systems. However, increasing capacity does not mean simply replacing the old system with a new one. In many instances, the capacity is increased through upgrading, i.e., adding more channels to the system without changing the old channels. Network operators prefer upgrading because it is less expensive than purchasing a new system and does not require network rearrangement. However, upgrading channel plans does not mean merely adding channels arbitrarily in the remaining available channel slots. With the increased number of channels and the decreased channel spacing, intensity-dependent nonlinear crosstalk in low dispersion optical fiber (dispersion-shifted fiber (DSF) and non-zero-dispersion-shifted fiber (NZ-DSF)) have become significant issues, in which FWM is of particular concern. Different channel plans yield different system performance and capacities. A desirable upgrade channel plan should include as many channels as possible while minimizing FWM crosstalk.
However, it is not an easy task to upgrade an old channel plan to a new channel plan with minimized FWM effect. Most current channel allocating schemes do not include the constraint that the new channel plan should include the old channel plan. As described above, the periodically allocating method obtains the channel plans by periodically allocating channels. However, this technique is hard to use for upgrading a channel plan since including the old channels may destroy the periodic allocation.
Another method to obtain unequally spaced channel plans, as described above proposes to minimize the FWM products that fall onto signal channels. This method uses a sequence as the channel spacings between channels in the desired channel plans. This method obviously is difficult to perform when upgrading, since manipulating the channel-spacing sequence is not compatible with the old channel plan that has fixed channel spacings between channels.
Recently, another new method was presented to obtain channel plans in minimizing FWM effect while considering fiber characteristics. In accordance with this alternative method, a channel plan is obtained by dropping those channels with maximum FWM crosstalk. This approach has yielded channel plans better than those obtained with the above-described methods. However, it can not be used for upgrading channel plan from an existing channel plan.
As noted above, an exhaustive computer search may be used to obtain a channel plan. This approach tries all the channel combinations for the desired channel number, of which the old channel plan is a subset. Again, performing this process is extremely difficult for WDM systems with large numbers of channels, because the number of possible combinations is so large for the given number of channels. For example, to upgrade from 11 channels 32 in a WDM system capable of carrying 96 channel total, 4.34xc3x971019 possible channel plans must be analyzed in order to determine the one with the least FWM crosstalk. This task is clearly prohibitive.
A technique in accordance with the present invention provides a systematic approach to find a sub-optimum channel plan with reasonable computational time. In each step, this technique allocates channels to minimize the FWM crosstalk in terms of fiber characteristics, while selecting as many channels as possible to increase the bandwidth efficiency. Since FWM crosstalk depends on fiber dispersion, channel spacing, channel power and number of FWM products, if FWM crosstalk is minimized, it means that the channel plan selected is optimal on all these parameters.